Differential amplifiers are utilized in a myriad of different applications in electronic circuits. At lower frequencies, the topology of conventional differential amplifiers typically takes the form of the so called “long-tailed pair” topology. With this topology, two input transistors each receive a respective input signal and the transistors have their source or emitter terminals interconnected and connected to a constant current source. This topology may not be suitable at higher frequencies due to instability of the amplifier and also due to a poor common mode rejection ratio (CMRR). A poor CMRR results in the amplifier being unable to adequately suppress spurious signals present on both the input signal supplied to the input transistors. Moreover, at centimeter and millimeter wavelengths, wideband or broadband differential amplifiers having relatively constant gain for input signals over a wide range of frequencies are typically difficult to design using the long-tailed pair topology.
Intermodulation distortion results from two input signals of different frequencies being mixed together due to nonlinearities in the transfer characteristics of a device receiving these two signals. High frequency amplifiers in multiple carrier broadband systems are such devices, for example. This mixing or intermodulation distortion of the two signals forms additional signals at frequencies that are not at harmonic frequencies (integer multiples) of either of the frequencies of the input signals, as well as at the harmonics. The largest intermodulation products appear at f1+f2, f1−f2, 2f1, and 2f2 and are referred to as second-order intermodulation while a third-order intermodulation occurs at 2f1−f2 or 2f2−f1 and has a lower power output power, as will be appreciated by those skilled in the art. Likewise, second and third order intermodulation are special cases of even and odd order nonlinearities respectively.
A differential amplifier with good common mode rejection should suppress the even order intermodulation distortion signals. If the differential amplifier has poor common mode rejection, however, then these signals may not be adequately suppressed and may accordingly be present at the output of the differential amplifier. In some applications where the bandwidth of the differential amplifier circuit is narrow these unwanted intermodulation distortion signals will be filtered out. In wideband differential amplifiers, however, the intermodulation distortion signals are not filtered out and must be suppressed in another way.
There is a need for an improved differential amplifier circuit topology.